The one update is that the posts here used the most recent publicly available version of our aggregate state legislative data. That data includes the years up through 2011. The Monkey Cage post uses a preliminary update to our data set, that brings the data to 2013.

We are working on polishing this data for public release shortly.

In the meantime, here are the new plots using that data set. The graph below shows legislative polarization in each state, averaging across all the years of our data (approximately 1996-2013) and across both legislative chambers. Polarization is defined as the average ideological distance between the median Democrat and Republican in the state legislature. Larger numbers indicate more division. The dashed line is the level of congressional polarization, included as a comparison (“US”).

About half of the states are even more polarized than Congress—which is saying a lot. At the same time, some states–like Louisiana, Delaware, and Rhode Island–have relatively less polarized state legislatures. In Louisiana, both parties are fairly conservative, and in Delaware and Rhode Island, they are both fairly liberal.

One state that stands out is California. It is incredibly polarized. (And its most recent primary and redistricting reforms look unlikely to reduce polarization.) Unlike Congress, however, Democrats both dominate the state so thoroughly and no longer need to attain supermajorities to pass budgets, so this polarization is not as much of an obstacle to actual lawmaking in the California state legislature.

Not only are states polarized, that polarization has increased over time. The graph below breaks down the trends in the ideology of Democrats and Republicans (measured by party medians) over time and across all 50 states. By convention, more positive scores represent more conservative preferences, and more negative scores represent liberal preferences. (One side-note: a data error exists in Washington State around the year 2000 and is being fixed.)

Most states have polarized over the past 20 years or so, but some more than others. Arizona, California, and Colorado are polarizing very fast. Nebraska—a state without formal political parties in its legislature—is polarizing very quickly too, though from a relatively low base.

Moreover, we are seeing asymmetric polarization, just as in Congress. Republicans have been getting more extreme faster than Democrats in more state legislative chambers, but this is by no means universally true across all states.

All in all, the picture we see in state legislatures is similar in many respects to Congress, but different in key points. The parties are pretty far apart on average, but that difference varies across the states. The parties are increasingly polarizing over time, but that too varies across state. Finally, we see cases of symmetric and asymmetric polarization. These new data on polarization at the state level—and the uneven pace of polarization across states—should help pundits and scholars figure out what’s driving polarization in our statehouses.